To prevent contamination of shallow freshwater sources, confine shallow gas zones and prevent the cave-in of unconsolidated rock formations, surface casing is inserted into the well when an appropriate depth has been reached. Cement is pumped up between the casing and the rock and drilling continues when the cement has set. A casing head is attached to the top of the surface casing.
When the drill bit reaches total depth and the well is to be placed on production, it must be cased to prevent it from caving in and to facilitate completion, production and maintenance operations. This production casing, after installation, has cement pumped between it and the annular space between the casing and the rock. The cement secures the casing in the hole and isolates the various zones from each other to keep the pressurized fluids in these drilled zones intact.
Because of the large weight of the production casing, a proportion, say 30-50%, must be borne by the casing hanger which is seated within the casing head. The casing hanger also acts as a seal to isolate the well pressure and well fluid or gas from the atmosphere.
Previous casing hangers have suffered from various disadvantages. On some, the wedge action in the casing hanger resulting from the weight of the production casing resulted in a prohibitively large compressive load. This force could cause a compressive failure in the production casing or a "bottleneck" resulting in reduced internal diameter which could hamper the insertion of various components and tools into the well.
A further disadvantage with previous casing hangers was that the seal used was not supported across its entire area. When testing occured to determine seal effectiveness following removal of the blow-out preventer, the seal could deflect in a cantilever type action which resulted in premature seal failure.
Yet a further disadvantage with prior casing hangers is that the seal action is not automatic upon release or lowering of the production casing into the gripping teeth. This made the seal effectiveness uncertain and in some hangers, a manual adjustment was required to obtain seal action after suspending the weight of the production casing from the slips and removing the blow-out preventer.
A further disadvantage in some casing hangers is that the seal is located beneath the gripper teeth which permits the flange seal test pressure to "add" to the slip imposed radial loads, thus increasing the possibility of "bottle necking" the casing.